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Average Atomic Mass

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**Chapter 7.4 Average Atomic Mass**

Objective: (1) To calculate the average atomic mass for an atom.

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Average Atomic Mass Weighted average of the masses of the existing isotopes of an element.

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**Recall: Atomic Mass Unit**

The unit that describes the mass of an atom Symbol: amu

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Example #1 The mass of a Cu-63 atom is amu, and that of a Cu-65 atom is amu. The percent abundance of Cu-63 is 69.17% and the percent abundance of Cu-65 is 30.83%. What is the average atomic mass of Cu?

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Example #1 The mass of a Cu-63 atom is amu, and that of a Cu-65 atom is amu. The percent abundance of Cu-63 is 69.17% and the percent abundance of Cu-65 is 30.83%. What is the average atomic mass? Step 1: Find the contribution of each isotope: Cu-63: (62.94 amu) x (0.6917) = amu Cu-65: (64.93 amu) x (0.3083) = amu Step 2: Add the relative abundances from each isotope together. amu amu = amu Round answer to two numbers after the decimal: amu

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Example #2 Calculate the average atomic mass of chromium. It is made up of isotopes with the following percent compositions and atomic masses: 83.79% with a mass of amu; 9.50% with a mass of amu; 4.35% with a mass of amu; 2.36 % with a mass of amu.

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Example #2 Calculate the average atomic mass of chromium. It is made up of isotopes with the following percent compositions and atomic masses: 83.79% with a mass of amu; 9.50% with a mass of amu; 4.35% with a mass of amu; 2.36 % with a mass of amu. Step 1: Find the contribution of each isotope: (51.94 amu) x (0.8379) = amu (52.94 amu) x (0.0950) = amu (49.95 amu) x (0.0435) = amu (53.94 amu) x (0.0236) = amu Step 2: Add the relative abundances from each isotope together. amu amu amu amu = amu Round answer to two numbers after the decimal: amu

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Example #3 Calculate the average atomic mass of iron if its abundance in nature is 15% iron-55 and 85% iron-56.

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Example #3 Calculate the average atomic mass of iron if its abundance in nature is 15% iron-55 and 85% iron-56. Step 1: Calculate the contribution of each isotope: Iron-55: (55 amu) x (0.15) = 8.25 amu Iron-56: (56 amu) x (0.85) = 47.6 amu Step 2: Add the contribution from each isotope together: 8.25 amu amu = amu The element with an atomic mass of 10.8 amu is Boron.

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Example #4 Chlorine exists as chlorine-35, which has a mass of amu and makes up 75.8% of chlorine atoms. The rest of naturally occurring chlorine is chlorine-37, with a mass of amu. What is the average atomic mass of chlorine?

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Example #4 Chlorine exists as chlorine-35, which has a mass of amu and makes up 75.8% of chlorine atoms. The rest of naturally occurring chlorine is chlorine-37, with a mass of amu. What is the average atomic mass of chlorine? Step 1: Find the contribution of each isotope: Cl-35: ( amu) x (0.758) = amu Cl-37: ( amu) x (0.242) = amu Step 2: Add the relative abundances from each isotope together. amu amu = amu Round answer to two numbers after the decimal: amu 1 – 0.758

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Example #5 Element X has two isotopes. One has a mass of 10.0 amu and an abundance of 20.0%. The other has a mass of 11.0 amu and an abundance of 80.0%. Estimate the average atomic mass. What element is it?

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Example #5 Element X has two isotopes. One has a mass of 10.0 amu and an abundance of 20.0%. The other has a mass of 11.0 amu and an abundance of 80.0%. Estimate the average atomic mass. What element is it? Step 1: Calculate the contribution of each isotope: Isotope 1: (10.0 amu) x (0.20) = 2 amu Isotope 2: (11.0 amu) x (0.80) = 8.8 amu Step 2: Add the contribution from each isotope together: 2 amu amu = 10.8 amu The element with an atomic mass of 10.8 amu is Boron.

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